The establishment of the Swedish Registry of Dementia in Twins, a unique resource, is proposed. The project has a dual purpose: (1) to carry out a study of genetic and environmental factors in the etiology and expression of demential of the Alzheimer's type (DAT) and other dementing illnesses in a defined population subsample, the Swedish Adoption/Twin Study of Aging, and 2) to refine methods of case ascertainment in preparation for expanding the study to the larger Swedish twin register. An estimated 80 living cases will be identified and worked up. Procedures will include psychological measures, medical evaluation, neuroimaging, and collateral interview encompassing family history and risk factors. Using register linkage and collateral interviews, an estimated 110 deceased probands will be identified. Co-twins of living and deceased probands will receive the same assessments as the living cases. Data analyses will answer the following questions: (1) What is the heritability of Alzheimer's disease? This question will be examined by determining concordance rates for MZ and DZ twins. Liability-threshold models will be used as well. (2) What roles do genes and environment play in the expression of DAT in genetically-susceptible individuals? Using case control methods, what risk factors such as environmental exposures can be identified? (3) What is the covariation of liabilities to different types of dementia? Do co-twins of individuals with DAT have a higher risk of other dementing illnesses as well as DAT? (4) What is the relative importance of sharing the same rearing environment for the expression of dementia? This question will be assessed by comparing concordances in twins reared apart and twins reared together. It is our intention to continue the investigation longitudinally in order to identify new previously unaffected cases, to chart rates of change in demented patients, and to follow-up co-twins of demented probands in order to detect early signs of dementia among this highly at-risk group.